GSK3β-FBXW7 axis regulates MAFB protein stability through ubiquitin-mediated proteasome degradation.

FBXW7, the substrate recognition component of the SCFFBXW7 E3 ubiquitin ligase complex, functions as a tumor suppressor by regulating cell cycle progression, stem cell maintenance, and cellular differentiation. Loss of FBXW7 contributes to hematopoietic malignancies and promotes monocyte-to-macrophage differentiation. Here, we identify the lineage-determining myeloid transcription factor MAFB as a previously unrecognized substrate of FBXW7. MAFB is a master regulator of monocyte-macrophage lineage commitment, terminal differentiation, and anti-inflammatory macrophage identity. We demonstrate that MAFB protein levels inversely correlate with FBXW7 expression during monocyte-to-macrophage differentiation in both THP-1 cells and primary human peripheral blood mononuclear cells. Consistently, FBXW7 depletion markedly increased MAFB protein abundance, establishing FBXW7 as a key regulator of MAFB stability during myeloid differentiation. Mechanistically, MAFB contains multiple conserved CDC4 phosphodegron motifs overlapping consensus GSK3β phosphorylation sites. FBXW7 directly associated with MAFB and promoted its ubiquitination and proteasomal degradation, whereas an F-box-deficient, ligase-inactive FBXW7 mutant failed to do so. Furthermore, in addition to previously reported GSK3β phosphorylation sites, our mutational analyses identified additional putative phosphodegron residues required for efficient FBXW7 recognition, although their direct phosphorylation by GSK3β remains to be established. Beyond myeloid differentiation, we observed an inverse relationship between FBXW7 and MAFB in multiple myeloma cells, where FBXW7 similarly destabilized MAFB. Collectively, our findings establish the GSK3β-FBXW7-MAFB axis as a conserved post-translational mechanism regulating MAFB stability in myeloid differentiation and multiple myeloma.